In order to secure that milk that is delivered to the consumers does not contain pathogenic microorganisms the milk is heat treated prior to the packaging. This heat treatment may be carried through at different time/-temperature combinations. Usual standards within this field are low pasteurization, high pasteurization and ultra pasteurization. Depending on the legislation in different countries the pasteurizing processes are different. As example of different processes there may be mentioned low pasteurization where the milk is heated to a temperature of 63-65.degree. C. and is kept at this temperature during 30 minutes or to 72-75.degree. C. with a holding time of 12-40 seconds. When high pasteurizing the treatment may comprise heating to 80-100.degree. C. during 10-20 seconds and at ultra pasteurization 110-125.degree. C. for 0.5-4 seconds.
A more forceful heat treatment gives a more complete killing of both pathogenic as other microorganisms present in the milk. The stronger the heat treatment the more pronounced is the effect on the chemical constituents, proteins, enzymes and vitamines in the milk which effect may be sensed as an impairment of the taste.
The shelf life of the packaged milk depends of course also on the treatment after the pasteurization step. It is extremely important to hinder reinfection during the process steps prior to the packaging in order to obtain good keeping qualities.
In order to obtain a milk product with a good keeping quality and a good taste one balances between the desire to achieve a product with as good bacteriological quality as possible, which product at the same time has not been effected negatively by the necessary heat treatment.
One way of obtaining a milk product with an extended shelf life is described in SE 451 791. According to this patent milk is divided into a cream and a skim milk fraction by means of centrifugal separation. The skim milk fraction is led to a microfilter and is divided into a permeate with a low bacteria content and a concentrate with a higher fat content than the permeate in the microfilter. The cream fraction and the concentrate are sterilized and remixed in desired proportions with the permeate after which the mixture passes a final heat treatment. The milk obtained according to the patent which is bacteria-poor may for example be used for producing cheese or be sold as consumer milk with extended shell life.